1. Field of the Invention
The present invention relates to an iron-based sintered alloy with dispersed hard particles, and more particularly, to an iron-based sintered alloy with dispersed hard particles suitable for a valve seat of an automobile engine.
2. Description of the Related Art
The combustion temperature of an automobile engine has been increasing as the power of the automobile engine increases, or as clean fuel such as LPG (Liquid Petroleum Gas) or CNG (Compressed Natural Gas) is used for reducing environmental load. Thus valve seats of engine components tend to be subjected to larger thermal and mechanical loads. To address the problems caused by the increased thermal load, materials such as, for example, chromium (Cr), cobalt (Co), and tungsten (W) are added to the raw material of an iron-based sintered alloy to enhance the strength of valve seats at high temperature. The strength required for the increased mechanical load can be enhanced by means of high pressure compacting, cold forging, powder forging, high temperature sintering, and the like. However, since the thermal and mechanical loads on a valve seat of an engine component are still increasing, it is conceivable that an engine will generate thermal and mechanical loads that conventional iron-based sintered alloys may not resist. For example, the thermal conductivity of the alloy can be enhanced through copper infiltration in which a low melting point material such as copper (Cu) is infiltrated into the internal pores of an iron-based sintered alloy, so that the thermal load on the valve seat can be reduced. However, the strength of the infiltrated iron-based sintered alloy is disadvantageously lowered by means of the infiltrated copper. In addition, secondary sintering is required to pack the alloy after primary sintering, thereby increasing the production cost.
As disclosed in Japanese Patent Laid-Open Publication No. Hei 5-93241, the present inventors have proposed iron-based sintered alloys having enhanced strength in which hard particles comprising molybdenum (Mo), carbon (C), and iron (Fe) are dispersed in an iron (Fe)-molybdenum (Mo)-nickel (Ni)-carbon (C) matrix. This publication discloses a technique for improving wear resistance through mixing boron (B) with the matrix to promote sintering and to form borides. Japanese Patent Laid-Open Publication No. Hei 9-53158 discloses iron-based sintered alloys with dispersed hard phase having enhanced strength through dispersion of hard particles comprising chromium (Cr), molybdenum (Mo), cobalt (Co), carbon (C), silicon (Si), and iron (Fe) in an iron (Fe)-molybdenum (Mo)-chromium (Cr)-nickel (Ni)-carbon (C) matrix, and also having improved wear resistance at high temperatures through formation of high-alloy phases through diffusion. Japanese Patent Laid-Open Publication No. 2000-73151 discloses iron-based sintered alloys with dispersed hard particles having improved wear resistance at high temperatures through dispersion of one or both of hard particle comprising chromium (Cr), molybdenum (Mo), cobalt (Co), carbon (C), silicon (Si), and iron (Fe) and hard particle comprising molybdenum (Mo), carbon (C), and iron (Fe) in an iron (Fe)-molybdenum (Mo)-chromium (Cr)-nickel (Ni)-vanadium (V)-carbon (C) matrix.
In an iron-based sintered alloy, a hard particle serves as a source of alloy elements, and also enhances deformation resistance at high temperatures. However, a hard particle such as a cobalt-based particle or a nickel-based particle serving as an alloy-source softens or hardens the alloy due to excessive alloying through diffusion of the alloy elements into a matrix. Also, a hard particle composed of intermetallic compounds, ceramics, carbides, oxides, and the like enhances the deformation resistance of the matrix, but has poor adhesive property (wettability) with the matrix, so that the hard particle tends to easily fall off the alloy matrix. The hard particles described above may deteriorate the wear resistance of the iron-based sintered alloy.
By dispersing hard particles of ferromolybdenum (Fe—Mo) composed of molybdenum and iron in a matrix containing silicon, nickel, molybdenum, chromium, vanadium, niobium, carbon, and iron, the wear resistance can be improved through a paving stone effect. However, since the diffusivity of molybdenum in the iron-based matrix is low, only the region around the added ferromolybdenum hard particles is strengthened, while the other regions are not strengthened. Further, since the bonding between the ferromolybdenum particles and the iron-based matrix is weak, the ferromolybdenum particles may easily fall off the iron-based matrix.